1. Field
The present invention relates generally to communications, and more specifically, to a system for conducting searches among asynchronous cell sites.
2. Background
The field of wireless communications has many applications including, e.g., cordless telephones, paging, wireless local loops, personal digital assistants (PDAs), Internet telephony, and satellite communication systems. A particularly important application is cellular telephone systems for mobile subscribers. As used herein, the term “cellular” system encompasses both cellular and personal communications services (PCS) frequencies. Various over-the-air interfaces have been developed for such cellular telephone systems including, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA), and code division multiple access (CDMA). In connection therewith, various domestic and international standards have been established including, e.g., Advanced Mobile Phone Service (AMPS), Global System for Mobile (GSM), and Interim Standard 95 (IS-95). IS-95 and its derivatives, IS-95A, IS-95B, ANSI J-STD-008 (often referred to collectively herein as IS-95), and proposed high-data-rate systems are promulgated by the Telecommunication Industry Association (TIA) and other well known standards bodies.
Cellular telephone systems configured in accordance with the use of the IS-95 standard employ CDMA signal processing techniques to provide highly efficient and robust cellular telephone service. Exemplary cellular telephone systems configured substantially in accordance with the use of the IS-95 standard are described in U.S. Pat. Nos. 5,103,459 and 4,901,307, which are assigned to the assignee of the present invention and incorporated by reference herein. An exemplary system utilizing CDMA techniques is the cdma2000 ITU-R Radio Transmission Technology (RTT) Candidate Submission (referred to herein as cdma2000), issued by the TIA. The standard for cdma2000 is given in the draft versions of IS-2000 and has been approved by the TIA. Another CDMA standard is the W-CDMA standard, as embodied in 3rd Generation Partnership Project “3GPP”, Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214.
Transmissions between component parts of a WCDMA system can be sent in a time division duplex mode (TDD) or a frequency division duplex mode (FDD), in accordance with the frequency bands available to a service provider. Due to the complexity of permitting operations in either mode, the system transmits information in accordance with logical channels and physical channels. (Logical channels are also referred to as transport channels in the WCDMA standard.) Data is encoded and interleaved according to the logical channel to which the data is assigned, and the logical channels are then mapped onto physical channels. The number and types of logical channels and physical channels vary depending upon the direction the signal is being sent. Transmissions from the mobile station to the base station are referred to as the “uplink” and the transmissions from the base station to the mobile station are referred to as the “downlink.”
Signals or data transmitted over a physical channel are carried in message entities, which are constructed using radio frames. Each radio frame comprises 15 slots and each slot corresponds to 2560 chips. A “chip” refers to a bit in a sequence formed after the original information signal is spread with a spreading code. Hence, each radio frame comprises 38,400 chips. However, message entities are variable in length since each message entity can be distributed and transmitted in a variable number of radio frames. In the current WCDMA standard, radio frames are designated as 10 ms in duration, and message entities can comprise 1, 2, 4, or 8 radio frames.
A WCDMA service provider may set up base stations in an asynchronous mode, such that each base station has an independent timing reference. In order to operate within the range of such asynchronous base stations, a mobile station must be able to acquire the frame timing of each base station with which the mobile station wishes to communicate. To receive and decode variable length message entities from a base station properly, the mobile station must first acquire the frame timing of the base station through an acquisition search for signals that convey the base station's frame timing information. Hence, if there are multiple base stations, the mobile station must conduct multiple acquisition searches for the timing of each base station with which the mobile station wishes to communicate.
The computational complexity and the amount of time required to conduct such acquisition searches can be extremely problematic for a mobile station that is traveling within the range of multiple base stations. In particular, if the mobile station moves from the range of one base station to the range of another base station, a delay in frame timing signal acquisition of the new base station can result in dropped calls. The process of maintaining a call while the mobile station travels from the communication range of a base station to another base station is referred to as a “hand-off.” Hand-offs can occur between sectors of a base station, between base stations of a single service provider, between base stations of different service providers, and between base stations operating at different frequencies. Hence, a traveling mobile station will likely experience the need to acquire frame timing information from multiple base stations. Unfortunately, the process of performing multiple acquisition searches quickly drains the battery life and processing resources of the mobile station.
There is a present need in the art to perform acquisition searches that will increase the likelihood of successful handoffs between asynchronous base stations, optimize the battery life of a mobile station, and minimize the allocation of processing resources within the mobile station. The embodiments described herein satisfy the aforementioned needs by implementing a system for conducting searches within new classifications of neighboring base stations.